Method and apparatus for measuring radioactivity of organic substance

ABSTRACT

A radioactive labelled compound is measured quantitively of its mass by a thermal conductivity detector (TCD) and then passed through a proportional counter to take radioactive measurement. As occasion demands, the compound is decomposed into CO2 and H2O and then H2O is trapped which is then reduced in a furnace by using carbon for preparing H2, on which quantitive measurement is taken by the TCD and then on which radioactive measurement is taken by the proportional counter.

United States Patent Saito et al.

[ 3,811,838 [451 May 21, 1974 METHOD AND APPARATUS FOR MEASURING RADIOACTIVITY OF ORGANIC SUBSTANCE [75] Inventors: Tomoo Saito, Sagamihara; Naotake Morikawa, Tokyo; Kazuo Wanatable, Yokohama, all of Japan [73] Assignee: Sagami Chemical Research Center,

Tokyo, Japan [22] Filed: Nov. 8, 1972 [2]] Appl. No.: 304,728

[30] Foreign Application Priority Data Nov. 10, 1971 Japan 46-089119 June I0. I972 Japan 47-057927 June I0, 1972 Japan.. 47-057928 [52] US. Cl 23/230 PC, 23/232 R, 23/253 PC, 23/253 R, 23/254 R, 73/23.l, 73/27 R [51] Int.Cl G0ln 31/08, G0ln 31/12 [58] Field of Search 733/27 R, 23.1; 250/374, 250/375; 23/230- R, 232 R, 255 R, 254 R,

230 PC, 253 PC [56] References Cited UNITED STATES PATENTS 2,499,830 3 1950 Molloy 250/374 3/1962 Fowlcretal. ..25o/374 4/l970 Simon ..23/232X OTHER PUBLICATIONS Anal. Chem. Vol. 32, No. 2, 2/1960, pp. 274-277, Sundberg et al., Application of Gas Chrom.

Primary Examiner-Richard C. Queisser Assistant Examiner-Stephen A. Kreitman Attorney, Agent, or Firm-Oblon, Fisher, Spivak, Mc- Clelland & Maier [57] ABSTRACT A radioactive labelled compound is measured quantitively of its mass by a thermal conductivity detector (TCD) and then passed through a proportional counter to take radioactive measurement. As occasion demands, the compound is decomposed into CO and H 0 and then H O is trapped which is then reduced in afurnace by using carbon for preparing H on which quantitive measurement is taken by the TCD and then onwhich radioactive measurement is taken by the proportional counter.

7 Claims, 12 Drawing Figures iSILIC AGEL PATENTEDMAYZI i914 sum 5 BF 9 A S WEE @ZFZDOU OOOw OOQq H0883 EEJVHHAV PATENTEBMAY 21 I974 SHEU 7 0F 9 Fig. O

A| BENZENE A TOLUENE B, BENZENE'4C B2 TOLUENE-C TIME PATENTEDMAY 2 1 I974 saw a or 9 Fig.

TIME

PATENTEDMAYZI I974 I I 3.81 l 838 sum 9 [1F 9 BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to method and apparatus for quantitative measurement of radioactivity of radioactive gas, and more particularly to method and apparatus for quantitative radioactive measurement of radioactive labelled compound or radioactive gases produced by dry combustion and/or reduction thereof.

This invention utilizes an improved gas-flow type proportional counter to be used in the radioactive quantitative measurement.

The invention also utilizes a reducing furnace which is particularly useful in the radioactive quantitative measurement of labelled compounds.

2. Description of the Prior Art Quantitative analysis of radioactive labelled compounds using H or C has increased its significance not only in the science of chemistry, but also in medical and biological sciences.

Liquid scintillation counting method and gas counting method have heretofore been used for the quantitative radioactive analysis, with a certain degree of success.

For measurement using spectral separation of H and C, particularly of double labelled compound containing H and C, it is known that the liquid scintillation counting has a very poor accuracy, except for limited cases in which specimens are free from quenching. In such measurement of H and C by the liquid scintillation counting, the amount of H required is usually about times or more of that of C. Even when an automatic oxidizing device of the specimen is used, the degree of separation of C and H is low and oxidizing efficiency thereof cannot be checked, and the use of such device results in an expensive cost increase for practical applications.

On the other hand, in the gas scintillation method, the simultaneous measurements of H and C of double labelled compound have not been performed'and it has been recognized to be difficult to measure even H labelled compound. l

Further, a measurement of labelledcompound has beenperformed by using CuO as an oxidizing catalyzer and Fe as a reducing agent. Such known measurement, however, has a shortcoming in that the life of the reducing agent is short and its reaction rate quickly diminishes and that hydrogen occlusion in Fe occurs so as to make it impossible to accurately measure hydrogen. With such gas counting method, its radiochemical yield of hydrogen is 60 percentat most and its radiochemical yield of carbon is 85 percent at most, and if only a very small amount of sample or specimen is used, the radiochemical yield of H may be reduced to a level of 20 to 30 percent.

Radio gas chromatographicapparatus is generally used together with the gas counting method, which apparatus continuously measures the radioactivity of label separated from the labelled compound by gas chromatography without decomposing the compound itself.- For example, the apparatus generally comprises a gaschromatography column, a detector, such as thermal conductivity detector, connected to the column, and a gas-flow type proportional counter coupled with the detector. A carrier gas is introduced into the apparatus, for instance, helium is introduced therein at about 10 to 30 ml/min. and a specimen is injected into the carrier from a specimen insert port. The mixture of the carrier and the specimen passes through the column,'where different components of the-mixture are separated. The seaprated components successively enter into a detector, such as a thermal conductivity detector (TCD), and the quantities of the different components are recorded on a chart as time elapses. After the components leave the detector, a counting gas is injected from a counter gas port, for instance, propane gas is injected at a rate twice as much as that of the carrier gas. The components are then introduced into a proportional counter, whereby the radioactivities of the different components are measured in succession. The radioactivities thus measured are usually plotted on a chart by the same recorder as they are measured by the detector. Thus, the radioactivities of the different components are represented by the outputs of the counter as peaks (curve A in FIG. 1) in the chart, along with the other peaks representing the output from the detector (curve B in FIG. 1), all of which peaks are drawn as deviations from a time axis of the chart.

The amount of the radioactivity for each component is usually represented by the number of decays per unit time. However, it is generally difficult to directly measure the amount of radioactivity of any moving gas as disintegration per time (dpm). The reason for the difficulty of the measurement is that the flowing gas travelling time necessary for passing through the effective volume of the counter tube is hard to determine, and that the time required for the gas to pass through the counter is changed even by minor variation of the flow rate of the gas therein.

According to a conventional practice, the radioactivity is determined by calculating the area in the chart for each of the aforesaid peaks such as shown by the curve B i.e. by integrating each of the peaks representing the radioactivities, but such calculation is very complicated and time-consuming. Above all, the accuracy of measurement by the conventional method is low.

For instance, in the case of a commonlyused counter with a volume of 10ml, if it is assumed that helium travels at a speed of 20 ml/min and-propane at 40 ml/min, the time necessary for passing such gases through the counter will be about 10 seconds. However, the effective volume of the counter tube, which affects the measurement, is not necessarily 10 ml, and accurate speeds of the helium and propane are hardly detectable. Furthermore, the flow of such gases in the counter may not be streamlined. Accordingly, there is a considerable dispersion in the time in which the gases pass through the counter. Further, even if an exact measurement of amount of flowing gas, the volume of sample or specimen and hence the amount of flowing gas can be varied when oxidizing or reducing the specimen, resulting in that the time for which the gas travels in the counter becomes unstable. As a result, the radioactivity of a certain component, which is determined on the basis of the above passing time through the counter, may have a considerably large error.

To mitigate such difficulty of the measurement, a standard or reference substance having a known radioactivity has been measured in reference with a specimen, so that the measured radioactivity of the specimen is expressed as a fraction of the total radioactivity of the standard substance, and/or as the counting rate of the specimen. With this method, the variation of the gas flow rate during a short period of time can be neglected, and the determination of the effective volume of the counter becomes unnecessary provided that the measuring conditions are very similar to those of the labelled compound. On the other hand, the actual counts for determining the radioactivity becomes small, because if it is assumed that the passing time is 10 seconds and 100% of the decays in the counter can be detected, the appare n t dyna nic counting efficiency is about 17 percent l /60).

To improve the apparent counting efficiency, the volume of the counter may be simply increased. The inventors, however, have found out that the increase of the counter tube volume would result in a reduction of the resolving power. What is meant here by the resolving power is the time duration from the beginning of the counting to the end of it. FIG. 2 shows this fact in which a curve A shows the output of the TCD as in FIG. 1 and a curve B shows the output of the conventional short i.e. small volume counter and B shows that of the counter of longer length of a larger diameter than those of the latter counter.

In view of such factors, counters having a volume of about 10 ml have been used, despite the comparatively low counting efficiency thereof. Thus, there is a need for a counter which has an improved accuracy and yet can provide a directly readable output representing the counted value directly.

As a portion of pre-treatment of measurement, oxidation and reduction are frequently used in analyses of single or multiple labelled compounds containing H and C using the gas counting method, and the reduction furnace is indispensable for the measurement of 3H- There are a number of different methods for reducing water, which can be roughly classified into the following two groups. Namely, (l) a process for quantitaf tively measuring heavy water in light water by using mass spectrography, and (2) a process using iron. The first process uses the following reactions.

W W203+H2 To effect either of the above reactions, the specimen water is placed in a vacuum vessel and allowed to stand for a long period of time for causing the reaction, and gaseous product formed thereby is collected by a suitable means, such as Teppler pump. This process may be accurate, but very complicated and not suitable for flow method or streamlined operation. The second process of using iron is commonly employed in radio gas chromatography, which is based on the reaction of Fe+H O FeO+H This reaction takes place at about 800C, and its reaction velocity is high and it is suitable for streamlined operation. Accordingly, the second process has been used fairly widely. The second process generally uses a furnace consisting of .a quartz tube and an electric heater, and the quartz tube has a CuO zone for oxidation, a quartz fiber zone, and an iron zone for reduction, which zones are disposed in the said order starting from the upstream. This furnace, however, has a shortcoming in that the life of the iron zone provided therein is short so that the reaction rate of the specimen therein is quickly decreased, and occlusion of hydrogen in the iron tends to occur so as to hamper the recovery of hydrogen. Due to such shortcoming, the quantitative analysis of H is hardly possible by such reduction with iron, as can be seen from theoretical and experimental studies thereof. In fact, experiment shows that the radioc hemical yield of H by the process of Fe-PH O H2+FeO is percent at the highest when dilution and washing with light water is included in the process, and the yield is barely 20 to 30 percent if simple reduction of water is effected by this process.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to resolve the aforesaid difficulties of the conventional liquid scintillation counting, by providing novel method and apparatus for measuring radioactivity of radioactive gas which is simple and by which the measuring accuracy is significantly improved.

To fulfill the aforesaid object, according to the method of the invention, an organic specimen is converted into carbon dioxide and water by dry combustion, the gaseous mixture thus formed is passed through a water trap for effecting quantitative analysis of the carbon dioxide alone for taking measurement on *C, the trapped water is brought out for quantitative analysis, and then radioactive measurement is effected on H.

Another object of the present invention is to provide a gas-flow type proportional counter for use in a radio gas chromatography in carrying out the aforesaid method, which counter provides a directly readable output representing the result of quantitative analysis and is highly efficient and capable of providing a high resolving power.

With the present invention, the apparent counting efficiency of a counter is improved by enlarging the volume of the counter. The enlargement of the volume is accomplished by using a thin but long passage through the counter, whereby, an integral type output from the counter is obtained while maintaining streamlined flow therein and minimizing the risk of turbulence therein. It is an object of the present invention to provide an improved reduction furnace, which is particularly suitable for flow-type process and yet free from the shortness of the life of reducing agent. The reducing furnace of the present invention can significantly increase the radiochemical yield of hydrogen.

DESCRIPTION OF THE DRAWING Other objects and advantages of the present invention may be more clearly understood by referring to the following description, taken in conjunction with the accompanying drawings, in which:

FIGS. I and 2 are already explained in which FIG. 1 shows typical outputs on a chart of a combination of the conventional TCD and proportional counter and FIG. 2 shows outputs of respective thin and thick proportional counters;

FIG. 3 is a schematic diagram, showing an embodiment of the present invention;

'FIG. 4 is a schematic diagram, showing another embodiment of the present invention;

FIG. 5 is a schematic sectional view of a furnace for reduction by carbon, which can be used for carrying out the method of the present application;

FIG. 6 is a schematic sectional view of another furnace for reduction by carbon, which is useful in the method of the present invention;

FIG. 7 is .a graph, showing the result of radioactive measurement by using a gas-flow type proportional counter tube with a thin but long passage according to the present invention, in comparison with similar result obtained by using a conventional proportional counter tube;

FIG. 8 is a graph, showing the accuracy of measure ment to be obtained by the method according to the present invention, in comparison with similar accuracy according to a conventional method;

FIGS. 9, 10, and 11 are graphs, showing the result of radioactive measurements by using a counter according to the present invention; and

FIG. 12 is a schematic diagram, illustrating the outcome of a measurement which is taken by a radio-elemental-analyzing method according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a preferred embodiment of the invention, as shown in FIG. 3, about 1 mg of radioactive labelled compound (*H', C) is introduced from a specimen inlet A into an electric furnace 12, which is a C/H/N automatic analyzer in this case, so as to convert the compound into CO and H 0 by dry combustion. (The advantage of using a C/H/N automatic analyzer as the electric furnace will be hereinafter.) hereinafter).

The converted CO N and H 0 are delivered into a cooling trap 16 wherein the H 0 is trapped by cooling, e.g., by dry ice. If necessary, a suitable carrier, such as He, is'injected through a valve into the converted compounds before the trapping. A three-way cock 18 is so set that the non-trapped fCO and N from the cooling trap 16 proceeds to a needle valve 20. The CO and N from the needle valve 20 pass through a silica gel column 24 in which these components are separated from each other, and then come to a thennal conductivity detector (TDC) 26, for quantitative measurements of the CO and N After the quantitative measurement, the *CO and N enter into a proportional counter 30, for instance, together with propane gas injected at a port 28. Radioactive measurement is taken on the CO in the counter 30, and then it is discharged through an outlet B.

Thereafter, the three-way cock 18 is turned for communicating the cooling trap 16 with a carbon reducing furnace 22, and the trap 16 is quickly heated so as to release the H 0 which is trapped therein. Thus, the released H O is delivered to the carbon reducing furnace 22, where it is converted into H and CO.

FIG. 5 shows a reducing furnace which can be used as the furnace 22 for the aforesaid conversion. The illustrated furnace comprises a porcelain tube 34 which is, for instance, made of alumina porcelain, and an electric furnace 50 surrounds the porcelain tuber 34 from the outside. Another porcelain tuber 32 extends into the aforesaid tuber 34 in a telescopic fashion. The top end ofthe porcelain tube 34 is sealingly closed by a lid 36 whichincludes an O-ring or other suitable sealing means. The lid 36 at the top end of the porcelain tube 34 has a gas outlet port 38. The bottom end of the porcelain tuber 34 is also closed by a closing means 40 which includes an O-ring or other suitable sealing means. A gas inlet port 42 is bored through the bottom closing means 40, as shown in FIG. 5.

The porcelain tuber 34 contains a procelain sand layer 48 disposed at the bottom portion thereof, and a platinum mesh layer 44 is overlaid on the porcelain sand layer 48. The lower end of the inner porcelain tube 32 is located in the proximity of the platinum mesh layer 44 with a spacing therefrom, and the top portion of the inner tuber 32 has a chamber for keeping carbon 46 therein, and the carbon 46 falls onto the platinum mesh layer 44 through the inside space of the inner tube 32 by gravity or in any other suitable fashion. Whereby, the carbon 46 is fed onto the platinum mesh layer 44.

In the present invention, carbon is used as a reducing reagent. The reason for using carbon as the reducing reagent has already been described in the foregoing. It may be added here that, of all the reducing reagents, iron has the lowest occlusion. However, iron is not free from hydrogen occlusion. Thus, in order to quantitatively recollect H the dilution process is indispensable. For the recollection by dilution, occlusion in gaseous oxide of carbon can be more easily destroyed than that in solid oxide of iron, so that hydrogen absorbed in oxide of carbon can be released in one operation.

The water with H as a label from the trap 16 is introduced into the porcelain tube 34 through the inlet port 42 thereof by using a suitable carrier, such as helium. Preferably, the H 0 may be heated prior to the arrival at the inlet port 42 of the porcelain tube 34. The porcelain sand layer 48 in the tuber 34 acts to increase the inside thermal capacity of the tube 34 after being heated by the electric furnace 50. The electric furnace heats the porcelain tube at about l,200C. After passing through the porcelain sand layer 48, the H 0 reaches the platinum mesh 44 and reacts with carbon 46 lying in the proximity thereof, so that it is reduced into H and CO.

If the amount of carbon 46 in the porcelain 34. for direct reaction with the H 0 is excessively large, the release of H absorbed in the incoming gas by occlusion sometime cannot be accomplished in one operation, so that the quantitative analysis thereof may become inaccurate. On the other hand, if the amount of such carbon 46 in the porcelain tube 34 is too small, the release of the H which is contained in the incoming specimen cannot be accomplished, and the quantitative analysis becomes difficult. Thus, there is a certain suitable range of the amount of the carbon in the tuber 34, and the presence of the carbon 46 in the tuber 34 at such range must be maintained throughout the analysis. In other words, the carbon must be fed into the tube at the same rate as it is consumed by the reaction in the tube.

The inventors have found that, for instance, in order to oxidize the H labelled compound of 0.5 mg to 2 mg and to release the occluded H in one operation by about 50 pl of light water, the suitable amount of the carbon is about mg to 600 mg. For ensuring the continuous presence of such suitable amount of carbon in the reducing furnace, the illustrated embodiment of the present invention uses an inner porcelain tube 32 which has a top chamber storing a supply of carbon and a lower end opening located in the proximity of the platinum mesh 44, so that a small gap is provided between the lower end of the inner porcelain tuber 32 and the outer porcelain tube 34 above the platinum mesh.

The carbon 46 in the top chamber of the inner porcelain tube 32 gradually moves down by gravity or in any other fashion, so as to make up for that amount of carbon which is consumed at the platinum mesh 44, for ensuring the aforesaid range of the carbon in the proximity of the platinum mesh throughout the analysis.

FIG. 6 illustrates another embodiment of reducing furnace which can be used in the present invention, while representing like parts by like numerals. This embodiment uses a quartz cotton or quartz fiber layer 52 between a porcelain sand layer 48 and a platinum mesh 44, for preventing the carbon from dropping into the porcelain sand layer. A cooling means 56 is provided between a lid 36 and an electric furnace 50, so as to prevent the heat from the furnace 50 from propagating toward other portions of the analyzing device. The principal reducing function of the embodiment of FIG. 6 is essentially identical with that of the embodiment of FIG. 5.

The H thus converted is completely absorbed by the carbon 46 in the reducing furnace 22. At the same time CO is generated, and this CO is delivered to the TCD 26 through the silica gel column 24 for measurement thereof. The quantitative measurement of CO facilitates the determination of the amount of H which is generated simultaneously therewith and occluded in the carbon of the reducing furnace 22. After the quan-.

titative measurement, the CO is discharged through the proportional couner 30 and the outlet port B.

Then, for instance, about 50 ul of light water is delivered into the electric furnace 12 through the inlet port A, and further moved to the reducing furnace 22. This light water fulfills the function of washing off all the H remaining at different parts of the passages to the furnace 22 by absorption or the like phenomenon, and to deliver such residual H O into the reducing furnace 22.

The H 0 and H 0 thus delivered into the reducing furnace 22 are converted into H and H respectively. A comparatively large amount of H from the 50 pl of light water, such as about 60 ml, acts to dilute the H which is previously occluded in the furnace 22, and at the same time all the H is expelled into the silica gel column 24. Thereafter, the amount of radioactivity thereof is measured by the proportional counter 30.

FIG. 4 shows another embodiment of the present invention, which is similar to the FIG. 3 except that the total amount of H 0 and CO coming out of an electric furnace l2 (Inclusive of N if N is generated) is measured by a TCD 31 immediately after the furnace 12. The use of the TCD 31 provides for the measurement of thetotal amount of gaseous substances at the electric furnace 12 before being forwarded to the trap 16, and the amount of water to be measured thereafter can be accurately determined, by subtracting the peak values of CO N and the like, which are separated by a gas-chromatographic column 24 and determined by a TCD 26, from such total amount.

According to the method of the present invention, the labelled compound coming out of the TCD 26 is passed into the counter which is characterized by having a thin but sufficiently long passage. The length of such thin but sufficiently long passage should be at least times of its sectional diameter, and the inventors have found out that the former should preferably be at least 30 times of the latter. If the length is shorter than 20 times of the sectional diameter, the measured output representing the radioactivity will not have a sufficiently large constant count portion.

The measurement taken by the counter having such a thin but sufficiently long passage will now be described, in conjunction with measured results obtained by conventional proportional counters and reference results obtained by using a proportional counter having a thin and long passage. These reference data were made by simple radio gas chromatography which was combined with conventional gas chromatograph apparatus and different shape flow proportional counter tubes.

FIG. 7 shows count-time relations for measurement of ethylene- C specimen by using different counters at room temperature, while passing helium at 20 ml/min and propane at 40 ml/min. The curve 101 represents the result with three series-connected counters each having a cross sectional area of 0.75 cm and a length of cm for providing a total inner volume of about 220 ml. The curve 102 represents the results obtained by using two of the same counters connected in series for providing an inner volume of about ml. The curve 103 represents the results obtained by using a 23 cm long counter with a cross sectional area of 7 cm for providing an inner volume of about ml. The curve 104 designates the results obtained by a commercially available conventional counter of [0 cm length and 1 cm cross sectional area. The curve 105 shows the peaks detected by the TCD 26.

The commercially available 10 ml counter, which corresponds to the curve 104 for the peaks of the TCD, had counting durations of about 10 seconds each and detected the instantaneous variation of the radioactivity concentration. However, the time for passing through the counter is short and the inner volume of the counter is relatively small, so that the counts obtained were small.

The inner volume of the counter which produced the curve 103 is substantially the same as that of the curve 102, but the curve 103 does not have a constant count portion and has only a very poor resolving power. Thus, it is apparent that the length and the cross sectional area of the passage affects results, even when the inner volume is kept the same. On the other hand, the curves 101 and 102 obtained by using counters each having a thin and sufficiently long passage have constant count portions, respectively. Thus, with the curves 101 and 102, quantitative measurement of radioactivity is possible, and the resolving power is considerably improved as compared with that obtainable by using a conventional counter or thick and long counters.

FIG. 8 shows mean error distributions, as determined by repeated measurement with a commercially available conventional counter having a small inner volume for producing the curve 104 of FIG. 7 and measurement with a counter having a thin and long passage for producing the curve 101 of FIG. 7. The solid lines in FIG. 7 represent mean errors for the case with the counter having the thin but long passage, while the dotted lines represent the mean errors for the case with the conventional counters with the small inner volume. As apparent from FIG. 8, the short counter causes dispersion of the measured values with an accuracy as shown by the dotted lines, while the counter of the present invention having the thin but sufficiently long passage cuts down the dispersion to less than one half of that of the conventional short counter.

FIG. 9 illustrates the result of measurement of a gaseous mixture of benzene-C and toluene- C, by using a counter consisting of three series-connected tubes each having a 95 cm length and 0.75 cm cross sectional area and a thin and sufficiently long passage as well as a commercially available HITACHI K-53 type gas chromatograph column ISE-30. The measurement was taken under the following conditions.

Carrier gas: helium at 15 ml/min Counter gas: propane at 30 ml/min TCD at 70 mA (150C) HV: 2,800 volts Time constant: 10 seconds Temperature of the counter with a thin but sufficiently long passage: lC

The curve A of FIG. 9 shows the peaks detected by the TCD, and the peak A1 represents benzene, while the peak A2 represents toluene. The curve B represents the count of the counter, in B1 represents the count of benzene- C, while the point B2 represents the count of toluene- C. As apparent from those curves, the counts for each component has a constant value portion and these constant heights indicate directly counts per time (cpt), therefore, the quantitative analysis and measurement for each component are facilitated. The curves also prove the high resolving power;

FIG. represents the result of other measurement, in which the peaks for benzene and toluene as determined by a TCD are close with each other. In the curve B, the peak Bl representing benzene- C and the peak B2 representing toluene-C are partially overlapped, and a high peak is formed therebetween. Nevertheless, there are clearly defined shoulder portions at both sides thereof, so that the quantitative radioactive analyses of such components can be effected despite of the closeness of the peaks therefor.

FIG. 11 shows the result of gas chromatograph analysis of a mixture of benzene, toluene, and methanol, by using a counter with a thin but sufficiently long passage according to the present invention. The peaks represent, from the left to the right, methanol, benzene, and toluene.

As apparent from the foregoing examples, the counter with a thin but sufficiently long passage according to the present invention has the following advantages.

Firstly, it produces constant count portions, so as to allow the determination'of the count at such constant count portion. This method is essentially different from a conventional process of determining the total area for each peak, and if the efficiency of the counter is known, the present invention provides a very easy method of the total radioactivity to measure its peak height. Furthermore, with the counter of the invention, the measurement is not affected by minor variations of the effective inner volume of the counter and the gas flow rate therein, and the resolving power can be improved by flowing a washing gas (e.g., propane) at any time after radioactive measurement or after a desired flat portion is obtained. The above and other characteristics of the present invention can be summarized as follows, in comparison with conventional devices.

Conventional Large volume Large volume counter counter counter (thin (thin and (thick and and sufficentshort long ly long passage passage) passage) Dimension lcm l0cm7cm 23cm 0.78cmX9ScmX2 Volume 10 ml 163 ml I48 ml Resolving power good poor good Counting rate small large large Influence by gas flow rate sensitive sensitive not sensitive fluctuation Absolute display difficult difficult easy of decay BG 30 cpm about 250 cpm about 250 cpm Data dispersion large large small Application qualitative qualitative quantitative (differential (integral type) type) FIG. 12 is a graph, which represents the result of the aforesaid tests with the device of FIG. 3 or 4. Triphenylmethane-C is delivered through the inlet port A, and converted into CO H 0 and N at the electric furnace 12 by dry combustion. The TCD 26 determines the amount of CO at first.

If another TCD 31 is used immediately after the electric furnace 12, as shown in FIG. 4, a peak representing the total volume of the CO and H 0 (inclusive of N if any) will be produced, as shown by the dotted line of FIG. 12. If nytrogen is present in the compound, the N is separated from CO in the silica gel column 24 for detection by the TCD 26 prior to the CO In FIG. 12, the curve T represents the measurement by the TCD 26, while the curve P represents the measurement by the special proportional counter 30 according to the present invention.

As pointed out in the foregoing, the "CO is detected by the TCD 26, and the corresponding peak is desig nated by the symbol CO in FIG. 10. Slightly after this peak, the radioactivity of the C in the CO is detected by the counter 30, which is represented by another peak "C in the figure. At the moment S, the three-way cook 18 is turned and the cooling trap 16 is simultaneously heated, so as to deliver the trapped H O from the trap 16 to the carbon reducing furnace 22. It is converted into H 0 and CO in the furnace 22, but H is occluded therein, so that only CO enters the TCD 26 for the measurement of the amount thereof. At the moment W, light water is injected, so as to dilute and wash off the H in the furnace 22 for sending it to the TCD 26, and the radioactivity of the H is then measured by the proportional counter 30.

Thus, with the method according to the present invention, if the C labelled compound is in the form of triphenylmethane, its radiochemical yield will be 100:1 percent, and if all labelled compound is in the form of triphenylmethane, its radiochemical yield will be 99% percent. In this case, the counting rate for C will be 92 percent and for H percent. Thus, the yield will be substantially 100- percent. Such performance represents a considerable improvement over conventional methods, because the conventionally highest yields for C and 1-1, as provided by the gas proportional counter of Packard Company of U .S.A., have been 75 percent and 58 percent, respectively. Furthermore, with the method according to the present invention, the conventionally cumbersome measurement of the amount of gas and its filling which are required in the conventional batch system are completely dispensed with.

In the reducing furnace using carbon according to the present invention, the amount of carbon which is present in the furnace is kept substantially constant I throughout the measurement. If the amount of carbon therein is too much, the recollection of the H by single dilution operation with light water cannot be achieved, while if the amount of carbon therein is too small, the desired occlusion of the H cannot be effected, as pointed out in the foregoing. If the amount of carbon in the furnace 22 is not kept at a proper level, the measured value of the radioactivity of the H will not produce a singular peak. The desired amount of carbon in the furnace 22, which is preferably maintained throughout the measurement, is experimentally determined. For instance, the inventors have found out that, in the case of oxidizing 0.5 mg to 2 mg of an organic compound and using 50 ul of diluting light water, the aforesaid desired amount of carbon in the furnace 22 will be about 100 mg to 600 mg.

The proportional counter to be used in the present invention has a very long passage, as compared with its cross sectional area. With such counter having a very long passage, the radioactivity can directly be measured as peak heights, and its resolving power is also improved.

As compared with the conventional liquid scintillation method in measuring double labelled compound containing ll and C, the method of the present invention can separately measure C and H regardless of the quenching phenomenon of specimens, and can provide a higher accuracy than that of spectral separation.

If an element analyzing device is used as an electric furnace in the method of the present invention, the analysis of the constituent elements of a compound can be effected simultaneously and continuously with the radioactive measurement for determining the structure of the compound. In this case, if the structure of the compound is known, the oxidizing efficiency can be checked.

The cost of apparatus for effecting the method of the present invention will be about one third of that necessary for carrying out the conventional liquid scintillation counting. Accordingly, the method of the present invention is very economical to carry out.

In the foregoing description, the present invention has been explained by referring to an example of using a double labelled compound of C and H. The invention, however, is not restricted to such example. More particularly, the present invention can be used for tracing a singular element or substance, or for measuring multiple labelled compounds including three or more labelled elements.

In the foregoing example of the reducing furnace using carbon, the amount of the carbon in the furnace is kept constant by a porcelain tube 32. The tube 32 may be made in the form of a cartridge, so as to simplify the replacement thereof. To improve the accuracy of the amount of carbon in the furnace, the porcelain tube may be vibrated for regulating the amount of carbon being made up through the tube.

The method of the present invention has been explained by referring to an application to radio gas chromatography, but the application of the present invention is not restricted to it. In fact, the present invention can be used for the radioactive measurement of various moving gas, such as radioactive measurement of a gas being burned.

What we claim is:

l. A method for measuring radioactivity of radioactive gas produced by dry combustion, comprising steps of converting a radioactive labelled compound delivered through a specimen inlet port into carbon dioxide and water by dry combustion, trapping the water for effecting quantitative measurement of the carbon dioxide inclusive of N if N is generated, by a thermal conductivity detector and radioactive measurement thereof by a gas-flow type proportional counter having a sufficiently long passage as compared with a cross sectional area thereof, releasing and delivering the trapped water into a reducing furnace using carbon for converting the water into hydrogen and carbon monooxide, measuring the amount of the carbon monooxide by the thermal conductivity detector while keeping the hydrogen as occluded in the furnace so as to determine the amount of the hydrogen thus converted, delivering light water into the reducing furnace through the specimen inlet port for washing off residual water adsorbed in intermediate passages and for diluting and expelling the hydrogen occluded in the furnace, and guiding the expelled hydrogen into the gas-flow type proportional counter for radioactive measurement thereof.

2. A method for measuring radioactivity of radioactive gas produced by dry combustion, as set forth in claim 1, further comprising a step, between said conversion of ratio-active labelled compound into carbon dioxide and water and said trapping of the water, measuring the total amount of the converted products by another thermal conductivity detector.

3. An apparatus for measuring radioactivity of radioactive organic substance comprising an electric furnace including a sample inlet for converting a labled compound into water and a mixture gas containing carbon dioxide, a cooling trap connected to said furnace for trapping the water, a silica gel column for separating components of the mixture gas passed through said cooling trap, a thermal conductivity detector for quantitatively measuring the components, a proportional counter for measuring the radioactivity of the components, a carbon reducing furnace connectable between said cooling trap and said silica gel column for reducing the water trapped in said cooling trap and feeding the hydrogen into said silica gel column to measure the radioactivity of the hydrogen after the measurement of the radioactivity of the components.

4. An apparatus for measuring radioactivity of radioactive organic substance as set forth in claim 3, further comprising a second thermal conductivity detector connected between said electric furnace and said cooling trap for measuring the total amount of the converted products from said electric furnace.

5. An apparatus for measuring radioactivities of organic substance in claim 3 wherein said carbon reduction furnace for reducing water into hydrogen and carbon monooxide by using carbon for radioactive measurement of the hydrogen, comprises a vertically disposed porcelain tube, a gas outlet port provided at top of said tube so as to communicate with the inside of the tube, a gas inlet port provided at bottom of said tube so as to communicate with'the inside of the tube, a porcelain sand layer filling lower inside portion of the tube, a carbon delivering means for delivering carbon onto the porcelain sand layer at a rate which is substantially identical with consumption rate of the carbon, and a heating means for heating at least that portion of said porcelain tube from outside which contains said carbon and porcelain sand layer.

6. An apparatus for measuring radioactivities of organic substances in claim 3, wherein said carbon reduction furnace for reducing water into hydrogen and carbon mono-oxide by using carbon for radioactive measurement of the hydrogen, comprises a vertically disposed porcelain tube, a gas outlet port provided at top of said tube so as to communicate with the inside of the tube, a gas inlet port provided at bottom of said tube so as to communicate with the inside of the tube, a quartz sand layer filling lower inside portion of the tube, a quartz fiber layer overlaid on said quartz sand layer, a platinum mesh means overlaid on said quartz fiber layer, a carbon delivering means for delivering rate on its output. 

2. A method for measuring radioactivity of radioactive gas produced by dry combustion, as set forth in claim 1, further comprising a step, between said conversion of ratio-active labelled compound into carbon dioxide and water and said trapping of the water, measuring the total amount of the converted products by another thermal conductivity detector.
 3. An apparatus for measuring radioactivity of radioactive organic substance comprising an electric furnace including a sample inlet for converting a labled compound into water and a mixture gas containing carbon dioxide, a cooling trap connected to said furnace for trapping the water, a silica gel column for separating components of the mixture gas passed through said cooling trap, a thermal conductivity detector for quantitatively measuring the components, a proportional counter for measuring the radioactivity of the components, a carbon reducing furnace connectable between said cooling trap and said silica gel column for reducing the water trapped in said cooling trap and feeding the hydrogen into said silica gel column to measure the radioactivity of the hydrogen after the measurement of the radioactivity of the components.
 4. An apparatus for measuring radioactivity of radioactive organic substance as set forth in claim 3, further comprising a second thermal conductivity detector connected between said electric furnace and said cooling trap for measuring the total amount of the converted products from said electric furnace.
 5. An apparatus for measuring radioactivities of organic substance in claim 3 wherein said carbon reduction furnace for reducing water into hydrogen and carbon monooxide by using carbon for radioactive measurement of the hydrogen, comprises a vertically disposed porcelain tube, a gas outlet port provided at top of said tube so as to communicate with the inside of the tube, a gas inlet port provided at bottom of said tube so as to communicate with the inside of the tube, a porcelain sand layer filling lower inside portion of the tube, a carbon delivering means for delivering carbon onto the porcelain sand layer at a rate which is substantially identical with consumption rate of the carbon, and a heating means for heating at least that portion of said porcelain tube from outsIde which contains said carbon and porcelain sand layer.
 6. An apparatus for measuring radioactivities of organic substances in claim 3, wherein said carbon reduction furnace for reducing water into hydrogen and carbon mono-oxide by using carbon for radioactive measurement of the hydrogen, comprises a vertically disposed porcelain tube, a gas outlet port provided at top of said tube so as to communicate with the inside of the tube, a gas inlet port provided at bottom of said tube so as to communicate with the inside of the tube, a quartz sand layer filling lower inside portion of the tube, a quartz fiber layer overlaid on said quartz sand layer, a platinum mesh means overlaid on said quartz fiber layer, a carbon delivering means for delivering carbon onto said platinum mesh means at a rate which is substantially identical with consumption rate of the carbon, a heating means for heating said porcelain tube from outside, and a cooling means disposed between said heating means and said gas outlet port.
 7. An apparatus for measuring radioactivity of radioactive organic substance as set forth in claim 3, wherein said proportional counter has a passage sufficiently long with respect to the diameter thereof, the ratio of the length of the passage to the diameter being sufficiently large to provide a direct reading of counting rate on its output. 